1. Field of the Invention
The present invention relates to devices and methods for inserting error correcting codes into data streams and for reconstructing data streams, as well as to corresponding products.
2. Discussion of the Background
The techniques for the continuous transmission of multimedia files, also known as “streaming”, permit Internet surfers to read these files in real time during the transmission, without having to wait for them to be downloaded completely. To do this, the data of the files are conventionally chopped into small elements so as to be transported from a server to a client in packets of smaller size than a critical size. This size, usually denoted MTU_size, represents the maximum size that the network can transport. It is determined both by equipment or hardware considerations such as the physics of the network (asynchronous transfer mode, denoted ATM; Internet protocol or IP; wireless communication, etc.), and by software considerations (routers constituting the network).
In practice, owing to the heterogeneity both of the routers (with different capabilities) and of the traffic (Internet transmission control protocol or TCP; user datagram protocol or UDP . . . with bit rates which are highly variable over time), an Internet network is subject to local congestions which give rise in particular to packet losses.
At the level of the client, associated with a decoder, the loss of a packet results in a discontinuity or several discontinuities (in the event of the use of an interleaving technique) in the data which it receives.
The decoder must then resynchronize itself on special words of the binary train (otherwise known as bit stream), such as image headers or resynchronization markers if they are present, as is the case in video transmitted according to the MPEG4 compression standard. The addition of these latter markers strengthens the robustness to errors (otherwise known as error resilience), but adds extra throughput, also known as “overhead” (around 25 bytes). Thus, the efficiency of the coding is reduced, thereby leading to its use being restricted.
Hereinbelow, the expression “resynchronization indicators” will designate indicators inserted into the data stream, and serving to perform a synchronization, preferably during the decoding process, whether it be a change of time in the stream (as with image headers) or a synchronization within one and the same image (hence in the course of one and the same time instant, as with resynchronization markers).
The TCP protocol makes it possible to remedy the problem of packet loss, by systematically retransmitting the lost packets which are not acknowledged by the client. However, this protocol is hardly appropriate to video streaming, since it introduces unacceptable values of retransmission lags and jitter. The UDP protocol is therefore preferred to it.
To protect UDP streams against losses, a widespread technique consists in the use of error correcting codes with no return path, or FEC (standing for Forward Error Correction codes), with data interleaving. Such FECs may in particular consist of parity codes or Reed-Solomon codes. They introduce a predefined level of redundancy making it possible to reconstruct lost packets.
Two types of such error correcting techniques are distinguished:                interleaving-based procedures with uniform protection or ELP-interleaved techniques (ELP standing for Equal Lost Protection), protecting each data packet uniformly,                    and interleaving-based procedures with variable protection or ULP-interleaved techniques (ULP standing for Unequal Lost Protection), protecting certain parts of the binary train more than others, depending on the type of data of the packets.                        
Another kind of method with variable protection is disclosed in document EP-A-0,936,772, which describes a ULP protection technique for audio coders. According to its teaching, n distinct classes are distinguished among the audio data, based on error sensitivity, that is, the impact of errors in those data, on the perceived quality of a reconstructed audio signal. A different level of error protection is allocated to each of the data classes. For example, in each data packet, one distinguishes bits related to side channels and bits related to a center channel for the stereo separation of the signal, or otherwise bits associated with predefined frequency ranges.
That method relies on a fine distinction of the belonging classes for the various bits of the audio information stream, which is implemented by means of a classifier able to distribute the data and to enable the definition of an error protection profile. That profile is then transmitted as control information to receivers.
However, reconstructions of part of the lost packets turn out to lead to a utilizable data size which is smaller than that which might be expected.
Specifically, when packets are rebuilt without it being possible to synchronize them appropriately, these packets are generally unusable. To reduce the risks of losing resynchronization indicators (such as image headers or resynchronization markers) , it is possible to apply the ULP-interleaved procedure while granting a higher level of protection to these indicators. The efficiency of the reconstructions nevertheless remains restricted, since any interruption in the chain of synchronized packets undermines the succeeding packets, until the next resynchronization indicator.
The additional application of a variable protection by means of a fine distinction between various kinds of data in each packet, as described in document EP-A-0,936,772, would significantly increase the implementation complexity without solving that difficulty.